I have an update manager icon. I can right-click it and select either "Check for Updates" or "Install all Available Updates". Both of these appear to do something useful. If I do the former first, it changes the icon to reflect how many updates are pending.

However, selecting "Update Manager" from that menu does nothing. Similarly, selecting "Software Updater" from the top system menu does nothing.

When I first installed 12.04, this wasn't happening. It started happening a couple of weeks ago.